Treachery of the Long Knives

Treachery of the Long Knives (Welsh: Brad y Cyllyll Hirion) was the reported massacre of British chieftains by Saxon mercenaries at a peace conference on Salisbury Plain in the 5th century. The story is part of both the Historia Brittonum and the Historia Regum Britanniae, as such it was widely known cautionary tale in Medieval Europe and has remained a significant part of both Arthurian legend and Welsh tradition.

"The Treachery" became a popular synonym for acts of deception both within Wales (Treachery of the Blue Books) and throughout the world. Most notably as the 1934 Night of the Long Knives.

According to the tradition, Vortigern, who had become the high king of the Britons in the wake of the end of Roman rule in Britain, allowed Anglo-Saxons under Hengist and Horsa to settle on the Isle of Thanet. He offered them additional provisions in exchange for their service as mercenaries against incursions by Picts and Gaels. The settlers, however, manipulated Vortigern into allowing them to increase their numbers and granting them more land, eventually including all of the Kingdom of Kent.

There is no specific account of this event in the 6th-century writings of Gildas. The story is known from the Historia Brittonum, attributed to the Welsh historian Nennius, which was a compilation in Latin of various older materials (some of which were historical and others mythic or legendary) put together during the early 9th century, and surviving in 9th-century manuscripts – i.e., some 400 years after the supposed events. According to John Morris's textual analysis of the Historia, this tale derived from a north Welsh narrative which was mainly about Emrys (Ambrosius Aurelianus), which the compiler of the Historia incorporated into a framework drawn from a Kentish chronicle, together with details from a Life of Saint Germanus.

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